1. Field of the Invention
This invention relates to a water-resistant surface coating formulation and also to paper coated therewith. More specifically, the present invention is concerned with a water-resistant surface coating formulation containing a reaction product of an acrylamide polymer and a glyoxalmonoureine, a method for improving the water resistance of paper by using the coating formulation, and coated paper so obtained.
2. Description of the Related Art
There is an outstanding desire for improvements in the water resistance of a surface coating formulation for paper, particularly for newsprint paper and printing paper, because they are printed by offset printing which requires dampening water.
Various surface coating resins have heretofore been employed to reduce separation of dust and/or filler from paper and/or to improve the surface strength and printing applicability of paper. Preferred conventional examples of such coating resins include starch, oxidized starch and modified products thereof as natural products, carboxymethylcellulose and hydroxymethylcellulose as semisynthetic products, and polyvinyl alcohol, polyacrylamide and derivatives thereof as synthetic products. Furthermore, many resins such as urea resins, melamine resins, water-soluble epoxy resins, styrene-maleic acid copolymers, polyvinyl acetate, vinyl acetate-maleic acid copolymers, latex resins and emulsion resins are used according to the application purpose. They are used either singly or in combination. Among these surface coating resins, starch, polyvinyl-base and polyacrylamide-base resins are employed preferably.
Although the above exemplified surface coating resins are significantly effective for obtaining paper having improved surface quality, they cause a variety of problems in relation to dampening water used at the time of offset printing. Examples of the problem include frequent paper break which occurs because a surface coating resin dissolved by dampening water sticks on rollers and paper winds itself around these rollers; an adverse effect on the control of release of paper dust; and irregular coating and deteriorated printing applicability attributable to localized dissolution of a surface coating resin.
To cope with these problems, it is the general approach to improve the water resistance of a coating resin. For starch or polyvinyl alcohol resins, it is the common practice to improve the water resistance by incorporating an epoxy resin or melamine resin. The starch resin having improved water resistance, however, requires a cumbersome dissolution step, and also involves other problems such as proneness to mildew and short service life after dissolution. The polyvinyl alcohol resins are also accompanied with problems such as irksome dissolution and troublesome post-dissolution foaming.
Polyacrylamide resins are also used after being mixed or being partially reacted with a urea resin, melamine resin, epoxy resin, zirconium salt, glyoxal, ketone resin, wax emulsion and/or various sizing agents. Japanese Patent Publication Nos. 13686/1965 and 26670/1969 (USP 3853816 et al.), for example, disclose a technique to react acrylamide with glyoxal, thereby improving the water resistance of the polyacrylamide resin. Japanese Patent Laid-Open No. 38705/1978 (USP 4122071, USP 4170672) and Japanese Patent Publication No. 18713/1981 disclose a technique to improve the water resistance of a polyacrylamide resin by using a glycidyl-containing monomer. When glyoxal or the glycidyl-containing monomer is added in an amount sufficient to improve the water resistance in the above technique, however, the coating resin so obtained is not stable in quality, for example, severe coloring occurs or its service life is too short.
Modification of coating resins with a chemical substance is also under investigation. Use of a formaldehyde-containing composition as a modifier should however be avoided in view of the troublesome odor of formaldehyde. From the viewpoint of environmental protection, it should also be avoided to use as a modier a chlorine compound which is a cause for dioxin contamination. Regarding paper quality, there is a demand for paper having high performance sufficient to meet diversified requests, for example, having superior water resistance without deterioration in surface strength and printing applicability.
It is the present situation that no surface coating resin having both long service life and superior water resistance has been developed yet.